Cutting insert with a threaded central aperture

ABSTRACT

A metal cutting insert includes a cemented carbide body having a cutting edge and a hole therein for receiving a fastening screw. The hole includes female screw threads for being screw threaded to the fastening screw. The insert is made by a process involving injection molding and sintering steps.

BACKGROUND OF THE INVENTION

The present invention relates to a cutting insert with an entirely orpartly threaded central hole for receiving a locking screw.

The vast majority of the plurality of different cutting inserts existingtoday on the market have a central through hole. This hole is intendedto accommodate a suitable locking screw for fastening the insert in aholder. If the cutting insert is intended for turning, the holderusually consists of a turning bar; if the cutting insert is intended formilling, the holder is, e.g., a milling cutter body, a long edge millingcutter or an end mill. The cutting insert may also be intended forboring and then it may be fastened by a screw in a boring shaft. Mostcommonly the cutting insert is an indexable cutting insert, which meansthat when a cutting edge has been worn out, the locking screw may beloosened and the insert turned by a half or quarter of a revolution,whereby a new cutting edge is placed or indexed into an operativeposition. Cutting inserts including indexable, cutting inserts existwith innumerous different geometries, in order to give a good chipbreaking and chip control and a low energy consumption. The inserts aremostly made of coated or uncoated cemented carbide, but cutting insertsof different ceramic materials also exist. They usually have a square,triangular, rhombic, rectangular or hexagonal basic shape, but otherbasic shapes also occur. However, they usually have one thing in common,namely that they are all provided with a central through hole in thegeometrical central point. These holes can have different hole designs,but they all have in common that the hole wall is substantially smooth.As mentioned, a threaded screw is inserted into this smooth hole and isthreaded into a threaded hole in the holder per se, upon which theinsert rests, and is fastened onto this holder. This basic configurationusually functions satisfactorily, but is nevertheless marred by somedrawbacks, which it would be desirable to eliminate. For instance, thescrew head must be countersunk into the insert's hole in order not toobstruct the chip flow, which brings about a further weakening of thecutting insert, which is already relatively fragile. Further,specifically because of this weakening of the insert's strength, thecentral hole cannot be made with too a large diameter, which means thatthe screw's diameter is relatively weak. Therefore, it sometimes occursthat the screw is broken when tightened with out care and/or toostrongly. Seen from another aspect, the small maximum allowed screwdiameter restricts the possible magnitude of the tightening strength.

Another disadvantage is that the requirement of accessability to thescrew head sometimes constitutes a complicating and restricting factorfor certain application e.g., for extremely close pitch cutters. Anothercase when good accessability is desirable is when a very large number ofcutting inserts are to be screw-fastened by a power-driven screwtightener with a preset maximal torque. For instance, in some barpeeling operations the number of cutting inserts is very high and theaccessability for such a screw tightener is poor.

Thus, one object of the present invention is to provide a cutting insertwith a larger hole diameter in order to make possible larger tighteningforces.

Another object of the present invention is to improve the accessabilitywhen tightening the locking screw.

SUMMARY OF THE INVENTION

These and other objects have been achieved in a surprisingly simple wayby providing a cutting insert with a wholly or partly threaded centralhole in accordance with the characterizing part of claim 1.

For illustrative but non-limiting purposes, the invention will now befurther described with reference to the appended drawings. These areherewith presented:

FIG. 1 shows in an exploded bottom perspective view the fastening of acutting insert according to the present invention.

FIG. 2 shows another embodiment of the fastening of a cutting insertaccording to the present invention, in a perspective view obliquely fromabove.

FIG. 3 shows another embodiment for fastening the cutting insertaccording to the present invention.

FIG. 4 shows a cross-section taken along line 4--4 in FIG. 3.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS OF THE INVENTION

In FIG. 1 a cutting insert, such as an indexable cutting insert, isgenerally designated by 1, and a holder by 2. The cutting insert is madeof one single piece. The geometry of the cutting insert 1 is notessential for the present invention, as long as the bottom supportsurface 3 partly or in its entirety is sufficiently planar to withstandthe stress and guarantee a stable cutting position.

The construction of holder 2 also is not an essential feature for thepresent invention, wherefore its depiction in the figures should beunderstood as purely principal. According to the embodiment as shown inFIG. 1, the holder is provided with a through-hole 4, which is intendedto accomodate a locking screw 5. The head 6 of the locking screw issuitably countersunk into the hole 4, the hole being shaped with alarger diameter in its lower part. Suitably, the hole is smooth in itsentirety. However, the hole 4 may also be provided with a threaded partcorresponding to the screw's thread 7, which is threaded past the threadof the hole and thereafter cannot fall out of the hole 4 any longer. Inthis case, the screw's unthreaded part 8 has to be of a somewhat smallerdiameter than the diameter of the thread 7, in order to rotate freely inthe thread of the hole.

The essential feature of the present invention is that the central hole9 of the cutting insert is threaded, in order to be fastened by thethread 7 of the locking screw, the thread of the insert being formeddirectly in the hard material of which the insert consists. Since theproduction of such a thread is not possible by conventional formpressing, the insert is produced by an injection moulding technique,which is previously known as such. This technique is a common productiontechnique in the plastic industry for the manufacturing of differentdetails. A paste consisting of or containing thermoplastic orthermosetting polymers is heated to a suitable temperature and is thenpressed through a die into a mould of desired geometry. In powdermetallurgy injection moulding is used for the production of relativelycomplex details of metallic powder, such as powder of cemented carbide.The cemented carbide powder is then mixed with polymers. After moulding,the polymer is driven off, whereafter the detail is sintered insubstantially the same way as corresponding tool-pressed details. Thethread is obtained by providing the mould with a threaded tap i.e., athread-forming element, in the middle, this tap being threaded out afterthe moulding, or being burnt away after the sintering.

The type of threads as such is not essential for the present invention.Thus, the central hole may be made with different nut (female) threadswhich are well-known per se, such as for instance a V-thread with atriangular basic profile (e.g., normal metric thread or so calledM-thread), different pipe threads, trapezoid and square threads, andalso so called round threads. Further, the hole may be tapered, althougha cylindrical shape is most common. The advantage with a conical threadis that the threads may be engaged and disengaged, respectively, by ashort turning, suitably less than two revolutions.

Preferably, the central hole 9 is a through-hole, but it may also be ablind hole. In the latter case, the length of the screw must of coursebe adapted thereto, in order to permit a sufficient play between thescrew's end and the bottom of the hole.

If the accessability from above the cutting position is good, aconstruction according to FIG. 2 may be chosen. In accordance with thisconstruction, the cutting insert 1' is fastened by a double-threadeddifferential screw 10. This screw has two threaded portions 11 and 12,respectively, and an unthreaded waist portion 13 between these twoportions. In a way known per se, these thread portions 11 and 12 arethreaded in opposite directions. For instance, if the portion 11 has aleft-hand thread then, also the hole in the holder 2' is left-handthreaded with, the thread portion 12 and the insert hole 9' beingright-hand threaded. The differential screw is rotated by inserting asuitable tightening device into the hole 14, which suitably is hexagonalor of the so called torx type. Also in this case, the hole in the holdermay be a through-hole, also the other end of the differential screwbeing provided with a suitable recess, for optional tightening eitherfrom above or from underneath.

Still another embodiment for the fastening of a cutting insert accordingto the invention is shown in FIGS. 3 and 4. According to these figures,a locking screw 15 is locked in a rotatable state by a securing washer16 with a threaded envelope surface 162. The holder is provided with ablind hole 17, which at least in its upper part is threaded for thethread-fastening of securing washer 16. However, before the securingwasher is threaded into the hole 17, the screw 15 is inserted throughthe central hole of the securing washer, whereby the screw is withheldin a rotatable state after the securing washer has been fastened. Thelocking screw is provided with a thread 18 which should be in theopposite direction relative to the threading of the securing washer. Theend of the screw is formed with a rotation-symmetrical recess 19 fortightening the cutting insert (not shown), the recess 19 in a usualmanner suitably being hexagonal or of the so called torx type. Ifdesirable, a hole of a smaller diameter than the diameter of the blindhole 17 may open into Lhe bottom of the latter hole and coaxially withit, the head of locking screw 15 being provided with arotation-unsymmetrical hole in substantially the same way as hole 19. Inthis way, the screw may be rotated either from below or from above.

As may be conceived from the above description, still another advantageof the present invention is based on the fact that the locking screw canbe withheld in the holder when replacing a cutting insert, and whenindexing a new cutting edge for indexable cutting inserts. In this way,the sometimes trying and time-consuming handling of small loose lockingscrews is avoided, which may easily be dropped and/or be difficult toset.

We claim:
 1. A method of making a cutting insert, comprising the stepsof:A. positioning a thread-forming element in a mold, the element havinga screw thread for forming a female screw thread; B. injection moldingin the mold a body from a mixture of cemented carbide powder and apolymer, the body having a cutting edge and a hole therein, at least aportion of a length of the hole having the female screw thread formed bythe thread-forming element; C. driving off the polymer; and D. sinteringthe remaining carbide powder.
 2. The method according to claim 1 furtherincluding, subsequent to step D, the step of burning away thethread-forming element.
 3. The method according to claim 1 furtherincluding, subsequent to step B, the step of unscrewing thethread-forming element out of the body.